The present invention concerns the field of radiological imaging. An object such as a human being or an animal or an object if a human being or animal to be studied under X-rays is placed between an X-ray source and a means of detection making possible a visualization of the X-ray beam after it crosses the object.
The control of radiological image exposure usually comprises keeping the brightness of the visible images constant. This principle is derived from the first constant sensitivity image receptors, notably, radiographic films for static images and fluorescent screens for fluoroscopy.
The known systems using indirect image receptors, based, for example, on image intensifiers, reproduce that behavior by having, for a given application, a fixed gain between the entrance exposure, also described as “entrance dose,” and the brightness of the image displayed, by adjusting the gains of the radiology apparatus, such as the optical gain by means of a diaphragm placed in the optical path, or an electronic amplification, or by adjusting a digital gain coefficient. The only exception to this fixed gain is to be found in case the X-ray parameters reach their upper limits on radioscopy. In such case, the gain is increased in order to compensate for decrease of the signal. This method is generally known in video systems as automatic gain control.
The method used for brightness or entrance dose control comprises using the signal supplied by a sensor, the signal being representative of the entrance dose or brightness, and comparing it to a reference corresponding to the desired level. The result of that comparison is entered in a device controlling the parameters of the X-rays used to obtain the image (supply voltage of the tube, supply current of the tube or product of the current by time), for the purpose of restoring the level desired.
It is of interest to observe what happens when the geometric enlargement is modified, that is, when either the object to be studies under X-rays, or the X-ray source or the image receptor is moved along the axis of the X-ray beam. By neglecting the diffusion effects of radiations, the brightness sensor will react exclusively to changes in distance between the source and the receptor, according to the inverse square of that distance.
If the distance is increased, a control loop will produce an increase in the X-ray parameters, and inverts it if the distance is reduced.
Now, this method can present problems. The distance between the X-ray source and the plane containing a significant detail of the object of interest and the image of which must be obtained is called SOD, and the distance between the X-ray source and the X-ray detector is called SID. If the enlargement ratio, equal to the SID/SOD ratio, is modified without, however, changing the SID, the number of X photons crossing the detail of interest will be modified according to the square of that ratio. Consequently, the amplitude of the quantum noise associated with the number of photons and, therefore, the effective signal-to-noise ratio will be modified, even if the modification of enlargement has no effect on the spatial resolution.
When the change of enlargement is made through a change of SID, the resulting effect on the signal-to-noise ratio will depend on the combination of effects of the change in intensity of the source caused by the modification of SID and enlargement respectively. In all cases, the change of geometries by displacement of the image receptor opposite the object and by displacement of the object in the direction of the source will entail a significant increase in the X-ray dose received by the object.